The present invention relates to a method for making a flexible and clear plastics material article of manufacture having a low surface electric resistance.
The invention also relates to the plastics material article of manufacture made by the method.
As is known, in making some flexible and clear plastics material articles of manufacture, for example inner liner of tubes for conveying fluid materials in general, it is necessary to provide the plastics material with electrical conduction properties, while preserving both the flexibility and clearness characteristics of the plastics material itself.
The present invention allows to modify, for example, the properties of high chemical resistance thermoplastic polymers such as polyethylene, polypropylene, polybutadiene, polyamide 6.6, polyvinylchloride, polyacrylonitrylebutadienestyrene (ABS), fluorinated polymers, such as polytetrafluoroethylene (Teflon), tetrafluoroethylene, hexafluoropropylene and vinylidene fluoride (THV) terpolymers, a tetrafluoroethylene and perfluoroalkylvinylether (PFA) copolymer, a tetrafluoroethylene and perfluoroethylvinylester (MFA) copolymer and a 1-propene, 1,1,2,3,3,3-hexafluoro-polymer with tetrafluoroethene (polyfluoroethylene-propylene, FEP), the latter being used for example for making clear inner liner or sheath elements of flexible hoses, but having such a high surface resistance, of the order of 1013 kΩ/sq, as to generate electrostatic charge accumulating problems, as they are used for conveying fluid materials.
A modification of the surface conductivity properties of the above mentioned materials has been made by applying on their surface carbon nanotubes (CNT), which represent an electrically conductive material with a very good chemical resistance and mechanical strength, compatible with the above mentioned polymeric polymers, while preserving the optical clearness and flexibility properties of the starting polymeric material.
Single and multiple wall carbon nanotubes CNT have been synthesized at the start of 1990. They represent a novel form of a nanostructured carbon based material (the single wall tubes have, for example, diameters of an order of a nanometer and lengths of an order of micrometers), which has mechanical strength, flexibility and electrical conduction properties which could not be found in other prior materials.
Carbon based materials, such as Carbon Black, are conventionally used for modifying the electric resistance of polymeric materials, into which they are included by a mechanical mixing and a subsequent melting of the composite material or by including them into the polymeric material as dissolved in suitable solvents.
The thus made materials, however, are, starting from surface resistances larger than hundreds kΩ/sq, not clear but deeply black, because their light absorption due to their carbon black contents.
On the other hand, said CNTs have the advantage that due to their unidirectional nature, they posses a percolating conductive limit at carbon concentrations much less than those which may be obtained from Carbon Black.
With respect to the carbon nanotube polymeric composite materials, in particular fluorinated polymers, the related literature is a very poor one and discloses only few works teaching to make carbon nanotubes composite materials from a solution for example of soluble fluorinated polymers such as Nafion, or by a mechanical mixing method, see for example B. J. Landi, R. P. Raffaelle, M. J. Heben, J. Alleman, W. VanDerveer, T. Gennett, Single Wall Carbon Nanotube—Naflon composite actuators, Nano Letters, vol. 2, page 1329, year 2002; J. Wang, M. Musameh, Barbon nanotube/Teflon composite electrochemical sensors and biosensors, Anal. Chem. Vol 75, page 2075, year 2003; K. El-Hami, K. Matsushige, Covering Single Wall Carbon Nanotube by the ply(VDF-co-TrFE) copolymer, Chem. Phys. Lett. Vol. 368, page 168, year 2003.
In particular the prior art in this field only teaches to include the carbon nanotubes by a mechanical mixing operation and a subsequent melting, or by dissolving the polymeric material into a solvent.
However, in such a method, the very high hydrophobic nature of a number of thermoplastics polymers and, in particular, fluorinated polymers, causes a strong interaction with the CNTs, with a consequent high difficulty in providing percolating path patterns as necessary for making a low electric resistivity composite material.
The above mentioned prior art shows that acceptable electric conductivities may be achieved only by an amount of CNTs providing a deeply black very expensive material.
The Applicants have found, from tests in which SWNT nanotubes were directly dispersed into a fluorinated polymer, in particular FEP, either molten or dissolved, that, this polymer does not allow to make a clear electrically conductive composite material, with an acceptable electrical conductivity and a sufficient clearness.
The scientific literature in this field, further discloses a possibility of making carbon nanotubes by filtering solutions in which said nanotubes are either dissolved or dispersed by dissolving or dispersing agents. For example, a method for making ultra-thin films, having a low electrical resistance and a sufficient clearness, has been shown in “Science (Transparent Conductive Nanotubes Films, Zhuan-chung et. al., Science, 305, 1273 (2004))”.
This prior method, however, comprises a plurality of complex operating steps such as to use dispersing agents and a depositing substrate to be successively eliminated, with a final transfer to an end support, and cannot provide high electric conductivity articles, such as the fluoropolymer based tube liners, even if the above authors have stressed the possibility of using said CNTs as unidimensional system for making good electrical conductivity ultra-thin films, and have attempted to design a simple industrial scale method to deposit a CNT based conductive material thin film on polymeric articles and firmly anchor this conductive film on the article surface, while limiting negative phenomena related to a deep embedding of said nanotubes into the polymeric material, for preventing in turn the electric conductivity of the article from excessively lowering.